The human body is capable of performing numerous activities that require varied amounts of force production. This can be achieved while using the same muscles to execute both fine and gross motor movements. The variety of activities that can be performed is immense, ranging from tasks of daily living to engaging in athletic competition. With this in mind, it would appear that the muscles comprising the muscular system have the ability to vary the degree of tension they generate, thus making it possible to engage in such a variety of motor tasks.
Two primary mechanisms have been documented to account for a muscle’s ability to vary its contractile force. First, an individual, via the central nervous system (CNS), adjusts the firing rate of neural impulses travelling down the motor neuron. This is referred to as rate coding. Second, again via the CNS, one can alter the number of motor units recruited to perform a task, hence the term motor unit recruitment. In either situation, it becomes apparent that the CNS plays a major role in determining the amount of force exerted by the muscle or group of muscles.
The identification of factors that regulate and contribute to force production may provide a greater understanding of the process of motor unit recruitment, thus aiding fitness and rehabilitation specialists in their design of effective training programs that will optimally stimulate and develop motor units utilized in tasks of daily living - ultimately enhancing quality of life.
It is the purpose of this article is to examine the scientific evidence presented and report on the patterns by which motor units are recruited and disengaged as muscle force increase and decreases respectively. To achieve this, it is important to have a general understanding of the underlying principles of neuromuscular physiology.
Your body has a remarkable ability to control the amount of tension exerted by skeletal muscles. During a normal contraction, tension rises smoothly and not in jerks due to the fact that activated muscle fibers are stimulated to complete tetanus. The total force exerted by the skeletal muscle is dependent on how many muscle fibers are activated.
Latash suggests that the CNS simplifies the task and decreases the computational load by uniting small elements of the neuromuscular system into functional units that are controlled with just one or two parameters. The smallest functional unit of the neuromotor system is termed a motor unit.
A typical muscle fiber is innervated by one neuron, called a motor neuron. The stimulus is an action potential sent from the motor neuron to the fiber. A motor neuron and the muscle fiber it innervates form a motor unit. The number of muscle fibers in a motor unit varies. More than 1000 fibers may comprise a motor unit in the lower leg, while a motor unit from the hand or eye may contain less than 100 fibers. As a rule of thumb, the fewer fibers in the motor unit, the more precise the movements.
The nervous system controls muscle force by controlling motor units. It has two options. The first is motor-unit recruitment, the process of varying the number of activated motor units. The second is rate coding, the process of varying the rate at which each active motor unit generates action potentials.
Motor-Unit Recruitment and Rate Coding
To recruit motor units means to activate them. They are usually activated and deactivated in a set sequence. The size principle is the most popular theory of how this orderly recruitment is controlled. Accordingly, the sequence depends on the size of the motor neurons. Motor units with the smallest motor neurons are recruited first and deactivated last. Motor units with the largest motor neurons are recruited last and deactivated first. According to the size principle, the order of recruitment is predetermined by size and is not directly controlled by the brain.
Muscle force depends not only on the number of active motor units but also on the rate at which motor units discharge action potentials. There are two kinds of rate coding. One method used by the CNS is to vary the overall frequency of action potentials. Another method is to vary the temporal pattern of action potentials between different motor units.
Rate coding through the control of overall action potential is the most complex. Generally, muscle force increases as discharge rate increases, but the relationship between muscle force and discharge rate is different for two types of motor units; tonic and phasic. Tonic motor units increase discharge rate as muscle force increases at low levels. Their discharge rate remains constant as muscle force continues to increase at high levels. In contrast, phasic motor units increase discharge rate as muscle force increases over the entire muscle force range.3
It is not known how these types relate to the classification of motor unit size (e.g., types S and FF). However, some other characteristics are known. Tonic motor units discharge smaller action potentials than phasic motor units. Tonic units are also recruited earlier and are less fatigable than phasic units.
Another form of rate coding is the discharge of two or more motor units at the same time. These synchronous discharges have been observed in muscles that were developed through strength training. It has been suggested, therefore, that muscle force increases when motor units are synchronized.
Although motor unit recruitment and rate coding have been discussed separately, these two options are executed at the same time resulting in muscle forces being determined by a combination of both kinds of control processes.
What Does the Research Indicate?
The manner in which motor units are recruited during gradation of the muscles force output have received considerable attention in recent years, with a number of studies examining motor unit discharge patterns. A large body of evidence indicates that there is a preferential sequence by which the motor units comprising a muscle come into play. This sequence has been described by the “size principle” of motor unit recruitment.
The Henneman principle (size principle) states "that the recruitment of motor units within a muscle proceeds from small motor units to large motor units." In general, the recruitment of motor units is governed by their size, with motor units being recruited in order from small to large. Small motor units have low thresholds of activation & are recruited first. Larger motor units with higher thresholds of activation follow this.
Motor unit recruitment is largely a function of effort. Fiber recruitment is determined not by the speed of the movement, but by the amount of force necessary to perform the movement. Small units are recruited when the load is light, while large units are not recruited until the load is heavy.
Fuglevand and Segal undertook a major investigation to determine the complexity of interaction between motor unit recruitment and oxygen delivery to muscle fibers. Normally, motor units are recruited in an orderly sequence, progressing from small to large force units. This property was also a feature of the model; recruitment proceeded in a fixed order from the unit innervating the fewest to the unit innervating the most fibers. As a motor unit was recruited, the locations of its fibers were registered on a computer display of the muscle cross section. They developed a computer model to simulate the perfusion of MVUs in response to different levels of motor unit recruitment. This was done to evaluate how differences in the spatial organisation of muscle fibers into motor units could influence the pattern and magnitude of capillary perfusion throughout a muscle cross section.
Results indicated when motor units were recruited in a sequence progressing from largest to smallest, only a few units needed to be active for most of the MVUs to be perfused. However, because these units innervated large numbers of fibers and because of the greater density of unit fibers in large than in small motor units (40 vs. 10 unit fibers/mm2), there was a sevenfold increase (P< 0.05 vs. control) in the number of active fibers (18.9 ± 5.9% of total fibers) needed to cause 90% of the MVUs to be perfused compared with when motor units were recruited in a small-to-large sequence. These findings suggest that, for sub-maximal contractions, muscle activated by electrical stimulation may have fewer perfused capillaries than muscle activated naturally for a given level of developed force.
The putative effect of motor unit recruitment order on MVU perfusion is not only of theoretical interest, but it may also be a consideration for clinical and experimental studies in which electrical stimulation is used to activate muscle. The threshold for activating an axon with extracellular electrodes is inversely related to the diameter of the axon. Therefore, when graded stimulation is applied to a peripheral nerve, large-diameter axons supplying large numbers of fatigable fibres will tend to be recruited before motor axons of small motor units innervating fatigue-resistant fibres.
A more recent study MacIntosh and Willis aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-µs duration supra-maximal pulses. Trials consisted of 8s of repetitive trains [5 pulses (quintuplets) two times per second or two pulses (doublets) five times per second] at 20, 40, 50, 60, 70 and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur.
When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction rest tension) of doublet contractions increased from 2.2 ± 0.2, 4.1 ± 0.4, and 4.3 ± 0.5 to 3.1 ± 0.3, 5.6 ± 0.4, and 6.1 ± 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 ± 0.2, 6.1 ± 0.5, and 8.7 ± 0.7 to 3.2 ± 0.3, 7.3 ± 0.6, and 9.0 ± 0.7 N, respectively. Initial peak active force values were 27 ± 1 and 61.5 ± 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz.
Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to pre-stimulation values by five minutes. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, de-recruitment or decreased firing frequency would be necessary to achieve a fixed (sub-maximal) target force during repeated isometric contractions over this time period.
In their study Griffin, Garland and Ivanova determined whether short interspike intervals (ISIs of <20 ms) would occur naturally during voluntary movement and would increase in number with fatigue. Thirty-four triceps brachii motor units from nine subjects were assessed during a fatigue task consisting of 50 extension and 50 flexion elbow movements against a constant-load opposing extension. Nineteen motor units were recorded from the beginning of the fatigue task; the number of short ISIs was 7.1 ± 4.1% of the total number of ISIs in the first one-third of the task (unfatigued state).
This value increased to 11.8 ± 5.9% for the last one-third of the task (fatigued state). Fifteen motor units were recruited during the fatigue task and discharged, with 16.4 ± 6.0% of short ISIs in the fatigued state. For all motor units, the number of short ISIs was positively correlated (r2 = 0.85) with the recruitment threshold torque. Short ISIs occurred most frequently at movement initiation but also occurred throughout the movement. These results document the presence of short ISIs during voluntary movement and their increase in number during fatigue.
This review has discussed some of the mechanisms by which muscle can increase its tension. One of these mechanisms, rate coding involves the rate at which neural impulses are conducted to the individual motor units. The second mechanism is motor unit recruitment, referring to the number of available motor units that are actually recruited.
The available research indicates that for movement to occur, specific groups of motor neurons in the spinal cord are stimulated. The contraction begins with the activation of the smallest motor units in the stimulated muscle. These motor units generally contain muscle fibers that contract relatively slowly. Over time, larger motor units containing faster and more powerful muscle fibers are activated, and tension production rises steeply. The smooth but steady increase in muscular tension produced by increasing the number of active motor units is governed by Henneman’s size principle. If the size principle did not hold true, every movement would involve an inefficient process, estimating how much force was needed to perform the task.
Henneman’s size principle ensures the most metabolically efficient means of completing any given movement. This is due to the fact that high threshold motor units, relative to low threshold motor units, are very costly metabolically due to their low oxidative capacities. The size principle guarantees that those large, high threshold motor units will only be recruited after smaller, more metabolically efficient motor units have been recruited.
In response to those who suggest that afferent impulses determine the sequence of motor unit recruitment, Deschenes is of the opinion that to rely upon afferent feedback would be ineffective in that it would require too much time for the CNS to process this afferent information and decide upon an appropriate recruitment pattern for the task.
At the completion of this review the author is of the opinion that there is convincing evidence to support the existence of the size principle of motor unit recruitment. However, this concept does have restrictions and should only be used within its proper context. The size principle can be verified in controlled, isolated instances when a muscle performs a simple motor task. The size principle may also be appropriate for muscles that are comprised of a homogenous distribution of muscle fiber types.
Understanding the process by which motor units are recruited may directly contribute to the designing of rehabilitation programs. In light of research supporting the size principle of motor unit recruitment, it appears not to be possible to design resistance training programs that develop large, fast twitch motor units exclusively.
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